GB2251902A

GB2251902A - A tandem master cylinder

Info

Publication number: GB2251902A
Application number: GB9101181A
Authority: GB
Inventors: Maurice John Wimbush
Original assignee: Automotive Products PLC
Current assignee: Automotive Products PLC
Priority date: 1991-01-18
Filing date: 1991-01-18
Publication date: 1992-07-22
Anticipated expiration: 2011-01-18
Also published as: GB9101181D0; GB2251902B

Abstract

In a tandem brake master cylinder having two pistons (14) (15) a spring (41) acts between the two pistons (14) and (15) and has one end seated on a spring seat (42) associated with a first piston (14) and its other end seated on a spring cap (43) connected to the first piston (14) by a screw threaded pin (44). The pin (44) screws into a tie bar (45) so that the overall length of the assembled first piston (14), spring (41), and spring cap (43) can be adjusted as required. <IMAGE>

Description

A TANDEM MASTER CYLINDER This invention relates to tandem master cylinders for hydraulic brake systems in particular for automobiles.

The present generation of master cylinders are generally manufactured with pistons having centre valves located in a passageway extending through the piston and which can allow fluid to feed from an unpressurised supply chamber to a pressure chamber.

With tandem brake systems in order to synchronise the operation of the pistons it is necessary to accurately located the two pistons with respect to the master cylinder bore and to each other. In order that the lost travel is reduced to a minimum it is known to have a tolerance compensating mechanism acting between the pistons to accurately position the two pistons relative to each other.

The present invention provides a compensating mechanism for a tandem master cylinder which is easy to assemble.

According to the invention there is provided a tandem master cylinder for a hydraulic brake system which has two pistons sealing slidably in a master cylinder bore to generate pressure in two chambers in the bore, both pistons being returned to an inoperative position by spring means including a first spring acting between the two pistons and having one end seated on a spring seat associated with a first piston and its other end seated on a spring cap connected to the first piston by a screw threaded pin, the first piston having a through passage therein in which a valve is housed, wherein the pin adjustably engages a tie-bar extending diametrically of the passageway at the end portion thereof adjacent the second piston, so that the overall length of the first piston, first spring, and spring cap can be adjusted as required.

The bar is preferably a transverse bar having end portions that are accommodated in axial slots in the end of the first piston, with an expanded centre portion to receive a thread forming screw thread on the pin, the bar being retaining in position in the axial slots by a circlip means.

The invention will be described by way of example and with reference to the accompanying drawings in which: Fig 1 is a longitudinal section through a master cylinder according to the invention, Fig 2 is a detail showing a plan view of a bar used to anchor the screw threaded pin to the first piston of the above master cylinder, Fig 3 is a detailed section showing the screw threaded pin as attached to the bar, Fig 4 is a section on the line IXJ-IV of Fig 3, Fig 5 is a detail section showing the detail of the valve opening means for the first piston of the above master cylinder, and Fig 6 is a section on the line VI-VI of Fig 5.

With reference to Fig 1, there is illustrated a tandem master cylinder for a vehicle hydraulic brake system, and which is compatible with brake systems associated with anti skid systems. The master cylinder 1B comprises a housing 11 having a master cylinder bore 12 with two pistons 1d, and 15 sealingly slidable therein. The master cylinder bore 12 is supplied with hydraulic fluid from a reservoir 16 via passageways 17 and 18 which feed into supply chambers 19 and 21 respectively disposed behind the primary seals of the pistons 14 and 15. (only the primary seal 22 of the piston 15 is illustrated).

The piston 14 located in the closed end of the master cylinder bore 12 (hereinafter called the second piston 14)is shown only in sufficient detail, for an understanding of the invention ana comprises a piston seal (not shown) is its end portion adjacent the closed end of the bore 12 with a centre valve 23 providing a connection between the supply chamber 19 and the pressure chamber side of the second piston 14, when the second piston 14 is biased into an inoperative position by a spring (not shown) housed in the closed end of the bore 12, against a stop pin 24 in a manner well known in the art. The stop pin 24 extends transverseley into the bore and is located in an axial slot 25 in the second piston 14 which allows the piston 14to slide axially relative to the stop pin 24.

The supply chamber 19 is sealed from the pressure chamber 26 of the other piston 15 (hereinafter called the first piston) by a seal 27. The second piston 14 has a smaller diameter end position 28 extending toward the open end of the bore 12 which has a coaxial bore 29 therein.

The supply chamber 21 of the first piston 15 is sealed from the atmosphere by a sealed closure disc 31 secured in position in the mouth of the bore 12 by circlip 32.

The first piston 15 has its primary seal 22 located between its supply chamber 21 and its pressure chamber 26. A co-axial stepped diameter passageway 33 in the first piston 15 interconnects the two chambers 21 and 26. The stepped diameter passageway has a larger diameter portion which opens into the pressure chamber 26 and which accommodates a valve 34. The head 35 of the valve 34 is located in the larger diameter portion 36 of the passageway 33 and has a co-axial stem portion 37 that extends through the passageway 33 for contact with an abutment 38 to lift the valve 34 and open the passageway 33. The valve 34 is biased to a closed condition by a valve spring 39 also housed in the larger diameter portion of the passageway 33.

The first piston 15 is biased towards the closure disc 31 by a return spring 41 which at one end abuts a spring seat 42 formed on the outer surface of the piston 15 and at its other end abuts a spring cap 43. The spring cap 43 is connected to the first piston 15 by a screw threaded pin, in the form of a set screw 44, which at its screw threaded end is fixed to a bar 45 secured to the piston diametrically of the larger diameter portion 36 of the passageway 33. The valve spring 39 bears against the tie bar 45 to bias the valve 34 to a closed position. The tie bar 45 is shown in Fig 2 and comprises a centre portion 46 having a hole therein to accommodate the screw threaded end of the pin 44, with radial lugs 47 on diamically opposite sides.

As is best seen in figs 3 and 4 the lugs 47 are received in axial slots 48 on the end of the first piston 15 and the tie bar 45 is then held in position by a circlip 49. The screw threaded pin 44 is preferably a thread forming screw which forms a thread in the hole in the tie bar. The cap 43 and screw threaded pin 44 are adjusted to compress the spring 44 so that the first piston 15, spring 44 and cap 43 form an assembly of a known desired length. As can be seen the spring cap 43 is a sliding fit over the end portion 28 of the second piston 14, and head of the screw 44 is accommodated in the bore 29 so that the spring cap 43 sits against the adjacent end face of the second piston.

The return spring 41 typically has a higher spring fitted load than the return spring (not shown) for the second piston so that the second piston return spring effectively returns both pistons to an inoperative position as a single assembly until the second piston abuts its stop pin 24.

Thereafter the first piston return spring will push the first piston 15 back against it abutment 38.

As can be seen in Figs 5 and 6, the abutment 38 comprises a cross member in the form of a bar 38 and a washer 62 in the mouth of the bore 12. The bar 38 is located in a hole 61 in the first piston 15 with sufficient axial play to allow the valve 34 to close.

The bar 38 in turn abuts the washer 62 to limit the movement of the first piston 15 towards the mouth of the bore 12.

The radially outer ends of the bar 38 are located within an annular shroud 63 fitted to the piston and held thereon by an inwardly projecting spring fingers or clips 64 engaging in a groove 65. The bar 38 is free to move within the shroud 63 which prevents the ends of the bar from damaging the internal surface of the bore 12.

When the first piston 15 is in its inoperative position the valve stem 37 abuts the bar 38 which causes the valve 34 to open thereby allowing the two chambers 21 and 26 to communicate.

The portion of the first piston 15 extending through the sealed closure disc has recess 71 therein to receive a push rod from a brake booster, or brake pedal to operate the master cylinder.

In the invention position is shown as Fig 1, both pistons 14 and 15 are biased against their respective abutments 24 and 62. The respective centre valves are open to supply fluid to the respective supply chambers 19 and 21. When the operating load is applied to the first piston 15 both pistons move into the bore 12, closing the valves, allowing a pressure build up in the pressure chambers for operation of the brakes.

Claims

1. A tandem brake master cylinder for a hydraulic brake system having a housing with two pistons sealingly slidable in a master cylinder bore in the housing to generate pressure in two chambers on the bore, both pistons being returned to an inoperative position by spring means including a first spring acting between the two pistons and having one end seated on a spring seat associated with a first piston and its other end seated on a spring cap connected to the first piston by a screw threaded pin, the first piston having a through passage therein in which a valve is housed, wherein the pin adjustably engages a tie bar extending diametrically of the passageway at the end portion thereof adjacent the second piston so that the overall length of the first piston, first spring, and spring cap can be adjusted as required.

2. A tandem brake master cylinder as claimed in Claim 1, wherein the tie bar is located in axial slots in the wall of the passageway, and has a centre portion that receives the threaded pin, and is held in position by a clip means.

3. A master cylinder a-s claimed in Claim 1 or Claim 2 wherein the tie bar acts as a spring seat for a valve spring which bias the valve to a closed position.

4. A master cylinder as claimed in any one of Claims 1 to 3 wherein the valve has a elongated coaxial stem that extends through the passageway in the first piston for abutment with a cross member which can open the valve against the bias of a valve return spring.

5. A master cylinder as claimed in Claim 4, wherein the cross member is a diameter bar located in a diametrical hole in the first piston with sufficient axial play to allow the valve to close under the bias of the valve spring, the pin being located in an annular shroud attached to the first piston to prevent ends of the pin contacting the master cylinder bore, and the cross member seating against an abutment fixed relative to the master cylinder housing when the first piston is in its inoperative position.

6. A master cylinder as claimed in any one of Claims 1 to 5 wherein the second piston has a coaxial smaller diameter extension thereon that projects towards the first piston, and the screw cap slidably fits over said extension.

7. A master cylinder as claimed in Claim 5 wherein the end face of the smaller diameter extension of the second piston has a hole therein to accommodate a head on the screw threaded pin.

8. A master cylinder as claimed in Claim 5 wherein the shroud has radical inwardly projecting spring fingers that engage in a slot in the first piston to secure the shroud thereon.

9. A master cylinder substantially as described herein and with reference to the accompanying drawings.